The present invention relates to a centrifugal separator for freeing a liquid from solid particles suspended therein and having a density larger than that of the liquid.
Several different types of centrifugal separators for this purpose are known, being arranged during operation to discharge continuously or intermittently sludge, which contains separated solid particles. Each one of the various types is intended for treatment of liquid containing particles in a certain amount and/or of a certain kind.
Thus, there are so called decanter centrifuges for treatment of liquids containing a relatively large amount of particles. An ordinary decanter centrifuge has a normally elongated rotor, in the separation chamber of which only one single conveyor screw is arranged to rotate around the same rotational axis as the rotor but with a different speed than that. Separated particles in this way may be transported by the conveyor screw axially within the rotor up to normally constantly open so-called sludge outlets. A decanter centrifuge of this kind is shown for instance in U.S. Pat. No. 4,245,777.
A decanter centrifuge of a somewhat different kind is shown in U.S. Pat. No. 3,685,721. This decanter centrifuge has several conveyor screws arranged in the centrifuge rotor separation chamber evenly distributed around the rotational axis of the centrifugal rotor. Each of the conveyor screws is arranged to transport separated particles to and out through a sludge outlet at one end of the separation chamber.
A centrifugal separator reminding of an ordinary decanter centrifuge, i.e. having only one conveyor screw, but having an intermittently openable so called sludge outlet, is shown in U.S. Pat. No. 4,508,530. In this centrifugal separator, which is intended for freeing a liquid from both solid particles heavier than the liquid and from liquid drops dispersed/emulgated in the liquid and being lighter than the carrying liquid, there is used for the separation also a set of conical separation discs which are arranged in the separation chamber radially inside the conveyor screw.
A decanter centrifuge as a rule has a large rotor volume and, thereby, a relatively large capacity, i.e. a large flow of a liquid containing a large amount of solid particles can be treated. The elongated rotor is normally journalled at both its ends and is adapted to rotate about a horizontal rotational axis. This means that a decanter centrifuge requires a relatively large floor area for its rotor, its driving means and its gear box, the last mentioned being needed for operation of the sludge transportation device within the rotor. Decanter centrifuges having centrifugal rotors rotatable around vertical axes are also known, however, for instance through U.S. Pat. No. 2,862,658. The rotor of such a decanter centrifuge is journalled either at its lower end or at its upper end.
Centrifugal separators of the kinds described so far are, as mentioned before, intended above all for treatment of large flows of liquid containing relatively large amounts of particles. For this purpose they have been given a construction which does not make large rotational speeds of the centrifugal rotor possible and which, therefore, are not suitable when there are requirements of substantial cleanliness of the separated liquid.
For such requirements of substantial cleanliness of the separated liquid there are other types of centrifugal separators. These are so formed, however, that they are not suitable for treating large flows of liquid having a large content of solid particles. Thus, there are so called nozzle separators of a kind shown for instance in U.S. Pat. No. 2,321,918. The rotor of a centrifugal separator of this type normally has a set of conical separation discs in its separation chamber and a rotor body the radial extension of which is in the same order as its axial extension. The reason for this form of the rotor body is that particles being separated from a liquid in the separation chamber should be able to move by themselves, i.e. slide on the surrounding wall of the separation chamber, without assistance from separate sludge transportation means, to sludge outlet nozzles situated at the surrounding wall of the rotor and being open or intermittently openable. The surrounding wall of the separation chamber in this case is constituted mainly by two conical rotor end walls, which are united with each other along their surrounding edges. A rotor having a design like this is expensive to produce and gets by necessity a rather large diameter without for this reason being usable for treatment of large amounts of sludge.
There are also centrifugal separators the centrifugal rotors of which in addition to conical separation discs in the separation chamber have equipment for intermittent discharge of particles from the separation chamber through separate openings or a slot extending all around the rotational axis of the rotor. The rotors for centrifugal separators of this kind are even more expensive to produce than rotors for nozzle separators and are even less suitable for freeing of liquids from large amounts of particles suspended therein. One example of a centrifugal separator of the last mentioned type is shown in U.S. Pat. No. 4,698,053.
The purpose of the present invention is to accomplish a construction for a centrifugal separator, which makes possible a design for the centrifugal rotor such that it gets a large separation efficiency, can be subjected to a large rotational speed, i.e. a large centrifugal force, and can be used for treatment of liquids containing small as well as large amounts of particles. The construction also should be such that the centrifugal rotor becomes relatively cheap to produce.
According to the invention this object can be achieved by means of a centrifugal separator which has a rotor having a center axis around which it is rotatable, the rotor comprising two axially spaced end walls and a surrounding wall arranged axially between the end walls and surrounding together therewith a separation chamber, the axial extension of which is substantially larger than the radial extension thereof, the end walls being adapted to be subjected to axially directed forces as a consequence of centrifugally generated pressure of liquid in the separation chamber, which forces strive to push the end walls away from each other; inlet means forming an inlet for introducing said liquid and particles suspended therein into the separation chamber; separation discs arranged in the separation chamber for creating small separation distances for the particles to be separated from the liquid; first outlet means forming a first outlet for discharging liquid having been freed from particles; second outlet means forming a second outlet for discharging particles having been separated from the liquid; transportation means comprising at least one transportation member which is situated within the separation chamber and is rotatable relative to the rotor, while this is rotating, for the transportation of separated particles axially in the separation chamber; transmission means for driving of the transportation means; and a central member extending between said end walls through the separation chamber and being dimensioned and connected with the two end walls in a way such that it will take up a substantial part of the forces acting axially on the end walls during the operation of the rotor.
In a centrifugal separator of this kind the centrifugal rotor may be given an axial extension which is large in relation to the diameter of the rotor, since separate transportation means is arranged for axial transportation of solid particles separated from the liquid. As a consequence of the force transferring connection between the rotor end walls and the central member the surrounding wall of the rotor need not transfer large axial forces caused by the pressure against the rotor end walls of the liquid present in the separation chamber during the operation of the centrifugal separator. Hereby, the connection between the surrounding wall and the end walls may be made relatively simple, since not even these connections need to transfer large axial forces. The rotor then may be produced relatively cheaply and still be designed for very large rotational speeds. This means that the invention makes it possible to produce a centrifugal separator able to separate large amounts of particles from a liquid and to separate the particles from the liquid rapidly and efficiently in that the separation discs, which may fill a large part of the elongated separation chamber, may be used in the best possible way.
Since the surrounding wall of the rotor need not be subjected to large axial forces in spite of a large rotational speed it can be produced from a thin material reinforced in the circumferential direction of the rotor by fibers of one kind or another, for instance carbon or glass fibers. The surrounding wall in this way may be given a large strength despite a small weight, which strongly contributes to making the rotor rotatable at a large speed.
For obtainment of the desired advantages of the invention it would be necessary that the central member, taking up axial forces, is arranged to take up at least half of said forces acting axially on the end walls during the rotor operation. However, preferably, the central member is adapted to take up substantially all such axial forces, i.e. more than 80% thereof.
Within the scope of the invention the rotor outlet for separated particles may be constantly open and provided with suitably dimensioned nozzles.
In a preferred embodiment of the invention, however, the rotor comprises outlet means adapted for intermittent opening of the rotor outlet for separated particles. Hereby, the outlet may be made large enough for safely avoiding clogging thereof, which is particularly important if some of the particles are substantially larger than others.
The above said transmission means for operating the transportation means in the rotor preferably is controllable in a way such that the axial transportation of separated particles in the rotor can be adapted to the actual content of particles in the treated liquid.
Even if the invention can be used also in combination with a sludge conveyor of the kind to be seen in an ordinary decanter centrifuge, e.g. according to U.S. Pat. No. 4,245,777, said transportation means preferably includes at least two conveyor screws journalled in the rotor and adapted to rotate around respective axes extending substantially in the axial direction of the rotor close to the surrounding wall of the rotor. In this case the separation chamber preferably has separate outlets for separated particles, each one of the conveyor screws being arranged to transport separated particles to one of the separate outlets.
If the centrifugal separator according to the invention has only one sludge conveyor in the rotor, extending helically around the center axis of the rotor along the surrounding wall thereof, it is desirable that the above mentioned central member takes up substantially all the axial forces caused by liquid in the separation chamber during operation of the rotor. Otherwise it is difficult to use the advantage of the invention that the surrounding wall of the rotor could be produced by a thin and light material reinforced by fibers extending substantially only in the circumferential direction of the rotor. However, if the centrifugal separator is of a kind having several sludge conveyor screws distributed around the center axis of the rotor the preconditions are somewhat different. In cases like that a number of bolts may extend between the two end walls of the rotor in the areas between the sludge conveyor screws radially inside the surrounding wall. In these areas there are normally arranged so called filler pieces, and said bolts preferably extend through these filler pieces.
Bolts of this kind may, if desired, be adapted to take up up to 50%, however preferably not more than 60%, of the forces acting axially against the end walls and caused by the pressure of the liquid in the separation chamber during operation of the rotor.
Alternatively, of course, the surrounding wall of the rotor, if desired, may be arranged to take up all or part of the axial forces which are not taken up by the above mentioned central member.
The separation discs in the separation chamber may be of any suitable kind. Preferably they are formed like truncated cones and stacked upon each other coaxially with the rotor. Alternatively, they may be formed such that they extend substantially axially through the whole or a part of the separation chamber and each of them extends arcuately from said central member towards the surrounding wall of the rotor, seen in a section through the rotor across the center axis thereof. In DE 48615 there is shown and described some different kinds of separation discs which may be used in a centrifugal separator according to the invention.